Volume control system for amplifiers



June29, 1937.

s. L. BEERS 2,085,068

VOLUME CONTROL SYSTEM FOR AMPLIFIERS File d May 1, 1930 2 Sheets-Sheet 1Source Ill INVENTOR George L Beers AT'TORNEY June 2, 1937. G. L. BEERSVQI-{UME CONTROL SYSTEM FOR AMPLIFIERS 2 Sheets-Sheet 2 Filed May 1,1930 .1 RF Sfaye 1 it flefecf.

P INVENTOR George L.Beers AT'TORNEY Patented June 29, 1937 ATENT OFFICIEVOLUBE CONTROL SYSTEM FOR AMPLI- FIERS Application Mlay 1, 1930, SerialNo. 448,863

6 Claims.

10 ception. This desirable result is obtained by the provision of acontrol tube having a plate-circuit resistor which is connected in thegrid circuits of a plurality of amplifier tubes of a radio receivingset, the positive end of the resistor being connected to the cathode andthe negative end to the grid of the amplifier tubes. The grid of thecontrol tube is connected to the grid of one of the tubes of the set insuch manner as to receive signal impulses, with the result that theplate current of the control tube will fluctuate accordingly. Anincrease in signal strength, which would ordinarily increase the volumeoutput of the loud speaker and possible overload certain of the tubes,will create a greater potential drop across the resistor in theplate-circuit ,of the control tube. This, in turn, will increase thenegative bias on the amplifying tubes and so cut down the signalstrength to the desired level. This action occurs simultaneously withthe incoming signal, with the result that the sound level, when onceset, is maintained substantially uniform, thereby correcting suchundesirable occurrences as fading, or the sudden booming which occurswhen switching from a comparatively weak station to a strong one.

The above described system functions Very efficiently in serving itspurpose of maintaining substantially uniform sound levels duringreception periods. It has been found, however, that,

during unusual circumstances, signals received tended to becomedistorted. These unusual circumstances occured at intervals when verystrong signals were received. The distortion-0f signals during thoseintervals of strong-signal reception is apparently due to applying a biggrid swing to the grid of an amplifying tube which is biased just beyondthe cut-off point, under which conditions, only the peaks of theincoming signals are amplified and the remainder of the signal lost,causing the signal to come through in an erratic manner.

To eliminate this undesirable feature, I have conceived the idea ofbiasing the first tube or tubes at a faster rate and to a greater extentthanthat of the subsequent tubes, accordingly varying thecharacteristics of the related amplifier circuits. Since the grid swingon the first amplifier tubes, as caused by an incoming modulated wave,is so small, as compared with that produced by the amplified signal onthe subsequent amplifier tubes, the grid bias on said first tubes may beshifted to practically any value, even beyond the cut-off point withoutintroducing, to any noticeable extent, the distortion referred to above.

This is not so, however, with the other amplifiers, wherein the gridswing approaches comparatively high values. To prevent distortionoccurring in these tubes, wherein the fairly large grid swing exists,such tubes should not be biased to the out off point, and thevolume-control grid-bias variations should not be of such value as tocarry the grid bias beyond the cutoff value.

Accordingly, by an arrangement for biasing the first amplifier tubes ata faster rate than the subsequent amplifier tubes, sufiicient controlcan be obtained without increasing the bias on the subsequent tubes tothe cut-ofi point.

My invention, as briefly outlined above, possesses additional utility ofa practical nature in that, in addition to its volume control functions,it might be utilized as a manually operated means whereby distortion ofthe type described above may be eliminated.

Accordingly, it is one object of my invention to provide a means wherebya uniform sound level in the output of a radio receiver may bemaintained during periods of reception, at any predetermined value, overa wide range of signal variations, such as occur during fading or whentuning from one station to another.

Another object of my invention is to provide a means for volume controlwhich will not introduce distortion on strong signals.

A further object of my invention is to accomplish the above effects in amanner which is entirely automatic.

An additional object of my invention is to provide means wherebydistortion may be eliminated.

Additional features of my invention will be pointed out in the followingdescription taken in conjunction with the drawings, which discloses aplurality of species of my invention, as applied to a receiver of thesuperheterodyne type, although it will be apparent that my invention isadaptable to receiving circuits of all types.

Fig. 1 of the drawings diagrammatically illusplification 8, and acontrol tube 9 whereby automatic volume control may be obtained.

A source of potential I I is provided for supplying the variousthermionic devices of the system with desirable voltages, said sourcecomprising any of the well known means of obtaining a direct currentpotential, e. g.: a battery, direct current generator, rectifier andfilter circuits or the like. An alternating current potential source,not specifically shown, is also provided for supplying current to theheaters l3, i5, l1, l9 and 21 and filament 23 of the tubes functioningin the circuit.

! While the system disclosed is more or less conventional, a briefdescription of the same is con- -25 sidered advisable to enable theoperation, of my invention to be more clearly and readily understood.

Referring more particularly to Fig. 1; the antenna circuit is showncoupled to the input circuit 25 of the radio-frequency amplifier 2, the

'output of which is heterodyned in the 1st detector stage to anintermediate beat frequency which is accordingly amplified through theintermediate-frequency stage of amplification 5. The 2nd detector 1 isprovided for detecting the output of the intermediate-frequencyamplifier 5, thereby reducing the intermediate frequency to anaudiofrequency which is capable of being further amplified by theaudio-frequency amplifier 8 provided for the purpose. Transformercoupling is used throughout the system but other means of interstagecoupling, such as resistance or choke coupling, may be resorted to.While I have illustrated only one stage of radio frequency andintermediate frequency amplification, it is obvious that a plurality ofsuch stages may be used. Although potentials of different values may beimpressed on all the tubes, I have illustrated, for the sake ofconvenience and simplicity, similar voltages onthe radio frequency andintermediate frequency tubes and, in like manner, the potentials on the1st and 2nd detector are made 7 similar. The cathodes of the radiofrequency and intermediate frequency tubes are, consequently; shownconnected in parallel arrangement to the point 27 on a voltage divider29, the grids of the same tubes being connected, in a similar manner,through different lengths of a resistor ill, to the point 33 on thevoltage divider 29, that portion of the divider. between the points 21and 33 providing a permanent bias on said tubes. Similarly, a portion ofthe voltage divider between the points 35 and 31 provides grid-biasingpotential for the 1st and 2nd detector tubes, for 5 it will be seen thatconnections from both points can be traced, by means of the conductor 39and ll, to the cathodes 43 and 45 and the grids 41 and 49 of both ofsaid tubes, respectively.

"Plate potential for the radio frequency and intermediate frequencystages is obtained from a common tap 5! on the voltage divider 29 and,in like manner, the detector tubes receive their plate potentials from acommon point 33 on the di- Vider.

Audio amplification is provided to comprise a aosaoes power tube 53coupled to the output circuit of the 2nd detector through a step-uptransformer 55; grid and plate potentials being supplied from points 33and 51 on the voltage divider, suitable connections (not shown) beingmade to the filament coil (also not shown) to complete the grid andplate circuits of said tube.

For adjusting the loud-speaker volume to any desired sound level andmaintaining said level substantially uniform, a control tube 9 isprovided. This tube is disclosed also as being of the heater type, but afilament type tube might be used in lieu thereof. A conductor 6|,extending between the grid 59 of the detector tube and the grid 63 ofthe control tube, renders both grids susceptible to the same signalinfluences. The low-tension side of the detector input circuit isconnected to the potential source at the point 37 by a conductor 42.Negative-bias potential is provided for the control tube 9 by connectinga portion of the voltage divider between the cathode at point 3? and thegrid of said tube at point 64 through a resistor 65. control tube 9 maybe fed from the same source as those of the other tubes.

The resistor 3| is also connected in the plate circuit of the controltube 9 between the plate and the source of potential. It will be obviousthat any change in the current in said plate circuit will produce acorresponding change in the potential drop across said resistor. It isthe drop across this resistor which is utilized in fulfilling theobjects of my invention and provides a very simple expedient forobtaining the desired results, as will be more fully explainedhereafter.

In the operation of the receiving apparatus disclosed in Fig. l, thegridcontact of the control tube 9 at the point 64 is adjusted to give adesired sound level at the loud speaker, and, as a result of myinvention, this sound level will be maintained substantially uniform,regardless of any variations in the strength of incoming Signals. Itwill be noted that, in addition to the permanent bias on the'radio andintermediatefrequency tubes, portions of the plate resistor 31 of thecontrol tube Salso constitute grid bias for the same tubes, more of theresistor being incorporated in the input circuit of the first tube thanin that of the second. Consequently, the

grid bias of the first tube will increase faster and r to a greaterdegree than that of the second tube.

Assuming an incoming signal to materially increase in strength, the gridof the control tube 9 will swing more positive, thereby causing anincrease in current in the plate circuitof said tube, with acorresponding increase in the potential drop across the resistor. As aresult, the grids of the radio-frequency amplifier tubes will swing morenegative, and, because of the greater portion of the plate resistorbeing in the input circuit of the first tube, the grid of the first tubewill be'made more negative than the grid of the second tube. Therefore,the conditions outlined above have been realized, and the distortionwhich would normally occur on strong signals will have been eliminated.

In Fig. 2, I have disclosed alternative means of accomplishing resultssimilar to those obtained by 'means of the species described inconnection with Fig. 1. Only the relevant parts of the circuit aredisclosed, the various elements of which bear the same referencenumerals as in Fig. 1.

In the modifications disclosed therein, it will be noted that the gridsof both amplifying tubes are connected to the same point on the plateThe filament 2! of the resistor 3|, instead of at different points, asdisclosed in Fig. 1. To obtain a relatively quicker and greatergrid-bias swing on the grid of the first amplifier, I make use of aresistor in either the plate circuit or the cathode lead of the lastamplifier; and I illustrate these alternative means by a switchingarrangement which is adapted, in one position, to connect the resistor61, in the plate circuit and, in the other position, to connect theresistor 69 in the cathode lead, as described. The effect of theresistor in the cathode lead is to reduce the negative bias on the gridof the last amplifier a substantial amount at the same time that theresistor 3| is functioning to increase the negative bias a greateramount, with the result that the grid receives an effective change dueto the difference of the two. A sudden increase in signal strength,therefore, will cause the grid of the first amplifier to swing negativeto a greater degree and at a quicker rate than that of the lastamplifier, since the first grid receives the full efiect of the resistor3|, whereas the last grid is only partially effected.

The effect of the resistor in the plate circuit is to increase the platepotential on the tube 5 as the bias on that tube is increased. Thisincrease in plate potential will, therefore, function to reduce theeffect produced by the increase in negative bias supplied by theautomatic volumecontrol tube. Therefore, while the ultimate effectproduced by the resistor in either the plate circuit or the cathode leadis the same, the manner in which the result is accomplished isdifferent.

While I have disclosed my invention as embodied in a radio receiver ofthe type described, it is obvious that my improvements might be adaptedto a receiving circuit of practically any type. A number ofmodifications of circuits are disclosed in my prior application referredto above, and the adaptation of my invention to any one of said circuitsshould be apparent. Accordingly, I do not wish to be limited to thespecific embodiment of my invention described herein except insofar asis necessitated by the prior art and the appended claims.

I claim as my invention:

1. In automatic volume control apparatus wherein two discrete potentialsare employed to control amplifier gain, means whereby one of saiddiscrete potentials is obtained by rectification of the amplifieroutput, and means whereby the second potential is derived from theourrent flow in an impedance located in the circuit connecting the anodewith the grid-to-cathode circuit of the amplifier.

2. The method of operation of an electrical wave amplifier so as tosecure automatic gain control thereof, which comprises applying to saidamplifier a gain control potential which varies automatically with themagnitude of the input of said amplifier and applying in the platecircuit only of said amplifier a second gain control potential whichvaries automatically as a function of said first gain control potential.

3. The method of operation of an electrical wave amplifier so as tosecure automatic gain control thereof, which comprises applying to saidamplifier a gain control potential which varies automatically with themagnitude of the input of said amplifier and applying to said amplifiera second gain control potential which varies automatically as a functionof said first gain control potential, said second gain control potentialbeing obtained by the voltage drop due to the flow of current in theanode circuit of said amplifier to the exclusion of the input circuit ofsaid 4. The method of operation of a carrier wave amplifier receivingelectrical signals of varying magnitudes so as to secure an automaticcontrol of the gain of said amplifier, which comprises impressing adirect current potential varying with signal strength upon the inputcircuit thereof and impressing upon the output circuit a second directcurrent potential automatically decreasing in magnitude as the strengthof the received electric signals increases, both of said direct currentpotentials functioning as gain control voltages.

5. In an amplifier system, a plurality of amplifying stages in tandemand means for changing the amplifier characteristics of certain of saidstages at a faster rate than those of subsequent amplifier stages inresponse to changes in carrier intensity whereby a uniform volume levelfor the system may be maintained without distortion.

6. In a system of the superheterodyne type having both radio frequencyand intermediate frequency amplification stages, the method ofmaintaining a distortionless transmission level which comprisessimultaneously varying the amplification of said radio frequency andintermediate frequency stages at different rates.

GEORGE L. BEERS.

